From precursor identification to the study of the distribution of 3-methyl-2,4-nonanedione in red wines and spirits

Abstract

Prematurely aged red wines are marked by intense prune and fig aromatic nuances that dominate the complex bouquet that can be achieved through bottle aging. This oxidation off-flavor is, in part, caused by the presence of 3-methyl-2,4-nonanedione (MND).1 It is interesting to note that similar aromas are also detected in aged spirits. Despite its strong sensory impact in red wines, the precursors of this diketone were not well described.

So, first investigations were performed in order to quantify this compound in young and aged spirits in order to explain these nuances. In addition, determination of MND precursors in red wines as well as the study of oxidation mechanisms on their evolutions will improve our ability to understand its formation pathway in alcoholic beverages.

The first step of this work aimed at identifying a precursor of MND in grapes and wines. Based on the MND distribution in grapes, in young and old wines we hypothesized that ketols might be precursors of this diketone. We describe the chemical synthesis of 2-hydroxy-3-methylnonan-4-one (syn- and anti-ketol) as well as their identification in wines. MND and ketols were quantified by SPME-GC-MS (CI, MeOH) using SIS and MS/MS mode, respectively, in more than 150 Merlot and Cabernet Sauvignon wines from California, Bordeaux, and Switzerland. Oxidation experiments conducted in model wine and red wine demonstrated that ketols are able to produce MND. Based on these data, their role as MND precursor will be discussed. In addition, we also report first results concerning the origin of ketols in grapes and wines.

We also extended our investigations to spirits were old samples can develop similar dried plum aromas. We also report for the first time the distribution of MND in many spirits including Cognac, Armagnac, Brandy, Bourbon, Grappa, Rhum, Whisky. Highest levels were found in grappa (> 10 000 ng/L), exceeding its detection thresholds (100 ng/L). Sensory analysis experiments revealed that this compound contributes to the aroma of spirits. Assay of ketols in these samples revealed that they can be precursors of MND in spirits.

This project has improved our understanding of the formation and evolution of MND in wines, enabling more accurate predictions of the oxidative behavior and aging potential of red wines. In addition, we describe its first identification as well as its sensory impact in spirits